编辑代码

<?php 
$arr = json_decode('[{"error_msg":"6931763140624","order_id":21793},{"error_msg":"6931763140631,6931763140624","order_id":21686},{"error_msg":"6937439916104","order_id":21566},{"error_msg":"6927164402867","order_id":21499},{"error_msg":"6931763140631,6931763140624","order_id":21316},{"error_msg":"6937439916104","order_id":21252},{"error_msg":"6931763140631","order_id":21153},{"error_msg":"6931763140631","order_id":21151},{"error_msg":"6931763140624","order_id":21024},{"error_msg":"6927164402867","order_id":21004},{"error_msg":"6927164402867","order_id":20858},{"error_msg":"6931763140631","order_id":20848},{"error_msg":"6931763140631","order_id":20571},{"error_msg":"6931763140631","order_id":20542},{"error_msg":"6927164402867","order_id":20526},{"error_msg":"6937439916104","order_id":20495},{"error_msg":"6974963012865","order_id":19378},{"error_msg":"4895235543950","order_id":18992},{"error_msg":"6974963012889","order_id":16883},{"error_msg":"6974963012865","order_id":14990},{"error_msg":"6974963012872","order_id":9880},{"error_msg":"6931763140631","order_id":4876},{"error_msg":"6931763140624,4955129027482","order_id":4580},{"error_msg":"6931763140631","order_id":4521},{"error_msg":"6931763140631","order_id":4362},{"error_msg":"6931763140631","order_id":4082},{"error_msg":"6931763170102","order_id":3893},{"error_msg":"6931763140631","order_id":3869},{"error_msg":"6931763140631","order_id":3770},{"error_msg":"6931763140631,6931763140624","order_id":3357},{"error_msg":"6931763140631","order_id":3221},{"error_msg":"6931763140631","order_id":2118},{"error_msg":"6927164402829","order_id":1912},{"error_msg":"6931763140631","order_id":1620},{"error_msg":"6931763140624","order_id":1592},{"error_msg":"6927164402829,6927164402867","order_id":1557}]', true);
var_dump(array_column($arr, 'error_msg'));
$res = [];
foreach(array_column($arr, 'error_msg') as $barcode) {
    $res = array_merge($res, explode(',', $barcode));
}

var_dump(json_encode(array_values(array_unique($res))));